1. Field of the Invention
The present invention relates to an electro-optical device in general. More particularly, it relates to such a liquid crystal display which can display clear and bright images at desired grades.
2. Description of the Prior Art
Liquid crystal materials can have anisotropy in permittivity, i.e. have different dielectric constants in the axial direction and directions perpendicular thereto. Because of this, the alignment of liquid crystal molecules can be controlled by applying an external electric field in order to influence the transmission of light incident thereon, e.g. for use in optical displays. Known examples of liquid crystals utilized include TN (twisted nematic) liquid crystal materials, STN (super-twisted nematic) liquid crystal materials, ferroelectric liquid crystal materials, polymer liquid crystal materials, dispersion type liquid crystal materials and so forth. It takes a certain time for such liquid crystal materials to respond to application of an external electric field. The responsive times, depending on the materials, are for example several tens of milliseconds in the case of TN liquid crystal materials, several hundreds of milliseconds in the case of STN liquid crystal materials, several hundreds of microseconds in the case of ferroelectric liquid crystal materials, several tens of milliseconds in the case of dispersion-type and polymer liquid crystal materials.
In order to construct visual images, a pair of polarizing plates have to be provided for display panels utilizing TN, STN or ferroelectric liquid crystal materials which are driven on the basis of birefringence. The maximum transmission of light through the panels is usually up to 20% to 30%. On the other hand, in the case of dispersion type liquid crystal materials, liquid crystal molecules having anisotropy in refractive index are confined within carrier in the form of capsules and arranged along the walls of the capsules. The liquid crystal molecules macroscopically appears to be arranged at random and cloudy in the capsules. Incident light in this case is scattered thereon. If an external electric field is applied to the dispersion type liquid crystal, the liquid crystal molecules are aligned in parallel to the electric field so that incident light can pass through the liquid crystal material without scattering. Such a liquid crystal panel requires no polarizing plate and therefore the maximum transmission reaches as high as 80% to 90%. In this case, however, switching elements such as non-linear devices or TFTs are needed to drive the panel because response of the liquid crystal itself to the applied electric field is not so good.
A usual existing active matrix type liquid crystal display is provided with active elements of amorphous or polycrystal thin film transistors of only one of p-channel or n-channel type. In general, an n-channel transistor (called NTFT for short) is connected to each pixel in series. The NTFT is located at an intersection of orthogonal signal lines in a matrix configuration of pixels and activated to transmit a signal to the pixel when control signals are given from the orthogonal directions.
It is, however, difficult to grade picture elements in hue and brightness in such an active matrix system. Conventionally, grading is considered to be realized by adjusting the strength of the electric field applied to respective pixels. Namely, an appropriate voltage is applied across a pixel from a peripheral circuit and transmitted to the pixel of the liquid crystal through a TFT given a gate signal.
Because of dispersion of characteristics of the TFTs and the matrix circuit, however, the voltage actually applied to the pixel tends to fluctuate at least several % or typically several tens %. On the other hand, voltage dependence of transparency of the liquid crystal is highly non-linear so that the transparency may significantly vary near a certain voltage level and even several % fluctuation can result in a rapid change of the transparency. Accordingly, it is impossible to realize grading beyond 16 grades.
In order to solve such a problem, the liquid crystal material is selected to have ability of sufficiently sustaining a voltage applied thereacross. The voltage, however, is gradually reduced because of a possible low resistance of the liquid crystal material or spontaneous discharge resulting in loss in charge accumulated across the liquid crystal material. It has been proposed, as illustrated in FIG. 1, to provide a capacitance 2 in parallel with each pixel 4 in order to compensate the loss by charge accumulated in the capacitance. The aperture ratio, i.e. the ratio of the effective displaying area of the liquid crystal to the entire area, is decreased.